In particular in the area of microlithography, apart from the use of components configured with a precision that is as high as possible, it is among other things desirable to position the components of the imaging device, in other words, for example, the optical elements (lenses, mirrors, etc.), the mask with the pattern to be imaged and the substrate to be exposed, as precisely as possible in relation to one another to achieve a correspondingly high imaging quality. The desired high precision properties, which are in the microscopic range in the order of magnitude of a few nanometers or below, are not least a consequence of the constant desire to increase the resolution of the optical systems used in the production of microelectronic circuits in order to advance the miniaturisation of the microelectronic circuits to be produced.
With the increased resolution and the reduction in the wavelength of the light used generally accompanying this, not only the demands on the positioning precision of the components used increase but the desired properties with regard to minimisation of the imaging errors of the overall optical arrangement are, of course, also increased.
In order to adhere to the high desired properties of the positioning of the components involved at the low operating wavelengths in the UV range used in microlithography, for example with operating wavelengths in the region of 193 nm, but in particular also in the so-called extreme UV range (EUV) with operating wavelengths in the region of 13 nm, it is frequently proposed that the position of the individual components, such as the mask table, the optical elements and the substrate table (for example a table for the so-called wafer) be determined, in each case individually with respect to a reference—for example a reference structure (that is stabilised with respect to its position, orientation and geometry against dynamic and thermal influences)—and that these components be then actively positioned with respect to one another.
Several problems occur here. On the one hand, it is generally desirable to achieve a positioning precision and therefore also a measuring precision of the measuring system used in the sub-nanometer range in up to six degrees of freedom. Likewise, the measuring system used has to be regularly calibrated to prevent the appearance of drifts, such as can occur during operation of an optical device of this type (for example due to thermal influences or the like). Consequently, it is desirable to achieve, eventually over several minutes through to several hours, high stability of the position of the components of the measuring system or to carry out a calibration correspondingly frequently. Finally, it is desirable to eliminate systematic errors of the measuring system (which are caused, for example, by a deviation of the components of the measuring system from their desired state).
If, for example, the position of a movable unit (such as the substrate table of a microlithography device or the like) is determined via an interferometry system, which is equipped with planar mirrors, as a position measuring device, planarity deviations of the planar mirrors (generally in the region of about 10 nm) have a direct effect on the measuring precision and therefore the positioning precision of the movable unit. In order to reduce systematic errors of this type (typically by a factor of 20), a calibration of the position measuring device generally takes place, as is known, for example, from EP 1 182 509 A2 (Kwan), the disclosure of which is incorporated herein by reference. A calibration of this type generally is repeated regularly at least over the time period during which the appearance of drifts (in other words, for example, thermally caused changes in the relative position of the components involved in the measurement) can occur.
Two different approaches are generally followed during a regular calibration of this type. On the one hand, it is known to equip the position measuring device with a corresponding redundancy, in other words to provide more than one measuring device for the respective degrees of freedom, in which a position measurement has to take place in order to identify and eliminate systematic errors via the redundant position information to be obtained in this way. However, this has the drawback that the outlay for the position measuring device can increase substantially as a high number of complex and relatively expensive highly precise measuring devices is involved.
A further approach involves the use of a so-called secondary reference element, which is permanently or temporarily connected to the movable unit, the position of which is to be measured during operation, as is known, for example, from U.S. Pat. No. 6,757,059 B2 (Ebert et al.) or U.S. Pat. No. 7,160,657 B2 (Smith et al.), the disclosures of each of which are incorporated herein by reference. To calibrate the position measuring device, the position of the movable unit is, on the one hand, determined by the first measuring device used during operation and, in parallel with this, by a second measuring device using the reference element. The reference element located in a precisely predefined position and orientation with respect to the movable unit in this case generally has an adequate number of reference marks (with a precisely defined position on the reference element), which are determined via the second measuring device (for example via a CCD camera) in order to determine the position and/or orientation of the movable unit via a corresponding processing of the measurement data of the second measuring device, as is known from U.S. Pat. No. 7,160,657 B2.